Electrically powered outdoor lighting systems are used to illuminate pathways, yards, parks and other like outdoor areas for security purposes. Commonly, such lights are connected to public utility systems or similar sources of electrical power, and are controlled by preset timing devices to illuminate desired areas at nightfall and automatically turn off at a predetermined time, for example, prior to daybreak.
Many conventional lighting devices require extensive cabling, suitable timing mechanisms and the like, and are thus relatively expensive to install and maintain. In addition, such lighting devices utilize electric power generated in a conventional manner such as by burning fuel. Burning fuel contributes to contamination of the environment and depletion of existing fuel resources.
More recently, self-contained solar powered illumination devices which utilize photovoltaic devices to charge batteries which, in turn, activate a light source contained therein, in the absence of sunlight, have been used for illumination and/or decorative purposes. Such self-contained devices are desirable because they are relatively inexpensive and require hardly any maintenance.
A plurality of such lights can be disposed in any predetermined arrangement, as desired, by pressing the light into the earth to position the lamp at a desired delineation or demarkation position. By thus positioning a plurality of the lights, a particular area, such as a pathway, may be easily delineated so that a person, even in complete darkness, may be able to follow the pathway without the necessity of producing sufficient light to illuminate the pathway.
Typically, solar powered lights include self-contained electrical storage devices, such as a battery which is maintained in a charged condition by a photovoltaic or solar cell array. Photovoltaic cells convert sunlight into electricity. A power supply electrical circuit controls the flow of current to a bulb contained within the solar powered light. The electrical power from the battery is supplied to the light bulb when the solar cell array is not producing electricity, that is, when the ambient light falls below a predetermined level and there is insufficient sunlight.
Although such solar powered illumination devices have been known to serve their purpose, they have not proven to be entirely satisfactory. During winter months, the run time of such solar powered lamps is about 30-35% of the run time obtained during the summer months because a large proportion of the sun's rays is reflected at the higher angles of incidence encountered during the winter. The same occurs at the beginning and end of a day, when the light levels are low and the angles of incidence are high.
Moreover, in present solar light constructions, the photovoltaic cells have substantially rectangular or square configurations. Thus, in circular solar lights there are substantial dead areas, such as at the four quadrants. Light impinging on these dead areas is not absorbed by the photovoltaic cell. Such lost radiation reduces the potential of current development by the solar powered light, which in turn affects the run time of the light.
In the past, because solar cells are expensive to produce, prior techniques have improved the efficiency of photovoltaic solar energy systems by attempting to focus incident sunlight from a large area onto smaller solar cells using a solar collector which incorporates optical concentrators. One such concentrator is a linear Fresnel lens. Because operation of such devices increases the radiant energy flux on the solar cells, the electrical current produced increases substantially. To efficiently carry this higher current, more and larger conducting elements are required. Such conducting elements are generally formed on the top of the solar cell itself, which prevents some of the sunlight from reaching the semiconductor material which converts the sunlight into electricity.
It is known in the prior art to provide covers for solar cells which refract incident sunlight away from conducting elements and concentrate it onto active cell areas. U.S. Pat. No. 4,711,972 to O'Neill discloses a solar cell cover geometry to improve the efficiency of solar cells used in a solar cell collector. The cover comprises a plurality of refractive prisms formed on the surface of the material, such prisms configured to refract incident sunlight onto small active cell areas rather than partially onto non-active conducting elements. Furthermore, the cover is configured such that a portion of the sunlight reflected from the cover itself is admitted into the cover and thereafter impinges upon the small active cell areas. The cover assembly of the O'Neill patent is also highly directional, requiring careful orientation and tracking relative to the sun.
Thus, a need exists for a means of effectively absorbing available radiation and increasing the run time of solar powered lights all year around and especially during winter months or at low ambient light levels.